The present invention relates to automatic recirculation valves and, more particularly, to recirculation valves directed to regulating the bypass recirculation flow in centrifugal pump systems.
Recirculation valves are frequently used in centrifugal pump applications to prevent pump overheating and maintain hydraulic stability. Pump overheating results from the transfer of heat energy created by the pump to the fluid flowing therethrough. During normal operating conditions (normal downstream demand for the pumped fluid) in a properly designed system, there is sufficient flow through the pump to absorb and carry away the transferred heat and thereby prevent overheating. During periods of low flow demand, however, the slower moving or even stagnant fluid absorbs a much greater amount of heat during its residence time in the pump, causing a substantial increase in the temperature of the fluid therein. As the temperature of the fluid within the pump increases, its vapor pressure increases, leading to cavitation which can damage the pump impeller and housing.
Low flow conditions can also result in a phenomenon classically known as internal recirculation. Under low flow conditions, hydraulic anomalies can occur within the pump. These anomalies are fluid responses to the less than optimum internal geometry of the pump at low flow rates and are generally initiated in the region where the fluid discharges the impeller near the pump housing discharge. This phenomenon, known as internal recirculation, results in cavitation which can damage the pump impeller.
Recirculation valves prevent pump overheating and maintain hydraulic stability by providing a secondary path through which the pump can maintain a sufficient fluid flow during periods of low downstream flow demand. One commonly used type of recirculation valve is a modulating flow valve as disclosed in U.S. Pat. No. 4,095,611 and U.S. Pat. No. 4,941,502. These patents disclose valves having an inlet, a main outlet, a recirculation outlet, a main flow element, and a bypass element with slotted orifices. Such valves are located downstream of the pump. Fluid enters the valve from the pump through the inlet, and exits the valve through the main outlet to satisfy the downstream demand. The recirculation outlet is connected to a secondary fluid path such as a low pressure reservoir or the pump inlet whereto the fluid is directed during periods of low flow demand in the main outlet.
The main valve element senses the rate of flow between the valve inlet and main outlet. During periods of normal downstream demand, a pressure differential across the main valve element causes the valve element to open and permit flow to the main outlet, while simultaneously causing the bypass valve element to close and prevent fluid flow to the recirculation outlet. Conversely, during intervals of low downstream demand, the main valve element returns to a closed (seated) position, thereby opening the bypass element and permitting flow through the recirculation outlet to the secondary path. Additionally, the main valve element, when seated, serves as a check valve which prevents reverse rotation of the pump impeller when the pump is shut down.
A problem associated with the use of such recirculation valves is that unbalanced fluid pressure forces acting on the bypass element may unsteady the bypass element's movement as the element approaches its open or closed position. This impairs the recirculation and checking performance of the valve. The bypass valve element comprises slotted orifices in the walls of a hollow shaft through which the fluid flows from the valve inlet to the recirculation outlet during periods of low flow. The orifices are opened and closed by modulating the hollow shaft relative to a stationary valve member that blocks the orifices when in the closed position. It has been found that the fluid forces exerted along the walls of the orifices are not uniform, particularly as the bypass element approaches the bypass opening or closing position. This results in a net force acting on the shaft that can unsteady its movement and impair the performance of the recirculation valve.
Accordingly, it is an object of the present invention to provide an improved recirculation valve.
Another object is to provide a recirculation valve with a smoother performing bypass valve element.
A further object of the invention is to provide a bypass valve element for recirculation valves that balance out fluid pressure forces acting on the orifices to provide more reliable performance.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.